New study questions the accuracy of satellite atmospheric temperature estimates
Posted on 7 November 2014 by John Abraham
Over the past decades, scientists have made many measurements across the globe to characterize how fast the Earth is warming. It may seem trivial, but taking the Earth’s temperature is not very straightforward. You could use temperature thermometers at weather stations that are spread across the globe. Measurements can be taken daily and information sent to central repositories where some average is determined.
A downside of thermometers is that they do not cover the entire planet – large polar regions, oceans, and areas in the developing world have no or very few measurements. Another problem is that they may change over time. Perhaps the thermometers are replaced or moved, or perhaps the landscape around the thermometers changes which could impact the reading. And of course, measurements of the ocean regions are a whole other story.
An alternative technique is to use satellites to extract temperatures from radiative emission at microwave frequencies from oxygen in the atmosphere. Satellites can cover the entire globe and thereby avoid the problem with discrete sensors. However, satellites also change over time, their orbit can change, or their detection devices can also change.
Another issue with satellites is that the measurements are made throughout the atmosphere that can contain contaminants to corrupt the measurement. For instance, it is possible that water droplets (either in clouds or precipitation) can influence the temperature readings.
So, it is clear that there are strengths and weaknesses to any temperature measurement method. You would hope that either method would tell a similar story, and they do to some extent, but there are key differences. Basically, the lower atmosphere (troposphere) is heating slower than the Earth surface.
In fact, for the time period 1987–2006, the temperatures among the four groups that collect satellite data ranges from 0.086°C per decade to 0.22°C per decade. In more recent years, the trend is much reduced, and for two of the leading satellite groups (University of Alabama at Huntsville and Remote Sensing Systems), temperatures are basically flat.
The recent flatness in satellite temperatures as surface temperatures continue to rise has presented a quandary for scientists. Are both results real? Is there some reason they diverge? Is one measurement more accurate than the other? This is one of the areas of very active research.
A contribution to this question appeared last week by researcher Fuzhong Weng and his colleagues. The paper, published in Climate Dynamics, claimed to find the reason for much of that difference – the authors report that the satellite trends could be off (too cold) by perhaps 30%. If true, this work would go a long way toward reconciling the differences between surface and satellite measurements.
While this paper is getting a lot of attention, I am suggesting a more cautious approach. There are a number of issues and questions which must be answered before we can close the books on this issue and the paper has received some critical attention from other scientists. Before we get into that, let’s talk about what the study found and how they made their discovery.
For a few decades, satellites have measured radiant emission from oxygen in the atmosphere and have related these measurements to temperatures. As satellites orbit the Earth, the microwave instrument on-board scans the atmosphere below them every 8 seconds or so and scientists apply what are called weighting functions to extract information from different altitudes. Each of the microwave “channels” uses a different weighting function so as to obtain information at different heights. The four channels most associated with atmospheric temperatures are Advanced Microwave Sounding Unit channels 3, 5, 7, and 9 in the current fleet of satellites.
The radiant emission received by the satellite can be influenced by other components in the atmosphere, in particular cloud liquid water. Many years ago, the impact of cloud liquid water was considered and various attempts were made to eliminate its influence through a filtering process. It is well known that cloud liquid water can influence the measurements, the real question is by how much?
The vigorous debate from the 1990s has been rekindled in the present Weng study. This new work segregates the Earth system by levels of cloudiness and precipitation in the atmosphere. The authors term “clear-sky” conditions corresponding to less than 10 grams of water per square meter of surface area. The authors then envisioned a cloud layer atop a raining region whose total height extends approximately 4 km vertically from the Earth surface.
In their analysis, they considered different droplet sizes ranging from .05 mm to 1 mm. Finally, the impact on the satellite channels (AMSU-A channels 3, 5, 7, and 9) was determined. It was found that the lowest channel (channel 3 which is primarily focused on the near surface region) was significantly impacted by the presence of cloud liquid water.
As you move higher into the atmosphere, the impact on temperatures was much reduced. When you look at the trends (change in temperatures with time), the two lowest elevation channels are higher when the impacts of clouds are removed. What this means is, measurements made in cloudy skies gives a lower warming trend of the atmosphere.
The authors state,
A decrease in brightness temperature can be associated with cloud and precipitation scattering, rather than physical temperature in the lower and middle troposphere and therefore, trends from microwave sounding data could be misleading if the brightness temperature from all weather conditions are averaged as representative of atmospheric physical temperature.
The trend calculated from the clear-sky data is thus not only larger but also more reliable ... It is shown that the atmospheric warming trends in the middle latitudes are significantly larger when cloud effects are removed … The scattering and emission effect of clouds and precipitation significantly reduces the values of the warm trends in the low and middle troposphere derived from microwave data.
Simply put, when you eliminate the effect of clouds, the atmosphere is warming faster than we thought and the divergence between land thermometers and satellites largely disappears.
Of course, whenever a study that is this significant is published, there is deserved skepticism. We have to be guarded in our acceptance until further work is done and until other teams have had a chance to review the findings. I asked others who work in this area to find their impressions.